I am neither an underwriter nor a broker, so I have no idea. In any case it would be difficult to provide a reliable number in a vacuum, as minimum requirements often vary according to various factors (fleet size, number of years claims free, etc.).
As an aside: it has been my experience that some flying schools tend to hide behind imaginary "insurance requirements" as a convenient excuse (e.g., "we'd love to rent you the 310 solo, but the insurance company says you have to have 500 hours on type first"). It is possible (I don't say probable) that you are being given a similar line as a way to get rid of you without causing an argument.